LTE (Long Term Evolution) Release 8 (Rel.8, or may also be referred to as “LTE”) defined in 3GPP (3rd Generation Partnership Project) and LTE Release 10 (Rel.10, or may also be referred to as “LTE-Advanced”) which is an enhanced version thereof introduce HARQ (hybrid automatic repeat request) which combines error correcting coding and automatic retransmission request (e.g., see NPLs 1, 2 and 3).
When HARQ is applied to transmission/reception of downlink data, a terminal performs error correcting decoding on downlink data transmitted from a base station, and then determines whether or not the data has been correctly decoded based on a CRC (cyclic redundancy checksum) added to the data. The terminal feeds back a response signal indicating an ACK (decoding success) or NACK (decoding failure) (hereinafter, referred to as “A/N signal”) to the base station based on the determination result. Upon receiving a NACK, the base station retransmits data in which an error has been detected.
An uplink channel such as PUCCH (physical uplink control channel) or PUSCH (physical uplink shared channel) is used to feed back such an A/N signal. Which channel is used to transmit an A/N signal or when an A/N signal is transmitted using PUCCH, which of a plurality of formats (PUCCH formats) is used to transmit the A/N signal is determined based on, for example, the presence or absence of uplink data when the A/N signal is transmitted, the presence or absence of CSI (channel state information), the presence or absence of a configuration of CA (carrier aggregation) or the like (e.g., see NPL 3). For example, when only an A/N signal is transmitted or when only a scheduling request for uplink data and an A/N signal are transmitted, PUCCH formats 1a/1b are used.
PUCCH formats 1a/1b are an uplink control signal format (most robust format) that can code-division-multiplex (CDM: code division multiplexing) 12, 18 or 36 A/N signals per frequency resource block (RB) and transmit/receive the A/N signals with the lowest required quality (required SINR (signal to interference and noise ratio)) (e.g., see NPL 1). PUCCH formats 1a/1b can simultaneously feed back A/N signals of 1 or 2 bits. For spreading for code-division-multiplexing in PUCCH formats 1a/1b, a ZAC (Zero auto-correlation) sequence having a sequence length of 12 is used for primary spreading, and a Walsh sequence having a sequence length of 4 and a DFT (discrete Fourier transform) sequence having a sequence length of 3 are used for secondary spreading. A/N signals transmitted by different terminals can be multiplexed by giving different cyclic shifts to a ZAC sequence in primary spreading or by multiplying the A/N signals by respective components of different orthogonal code sequences in secondary spreading. Note that an orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. The orthogonal code sequence may also be referred to as “block-wise spreading code.” In the following description, one PUCCH formats 1a/1b resource determined by an RB, a primary spread cyclic shift and a secondary spread orthogonal code sequence is referred to as “A/N resource” or “PUCCH resource.”
In order to code-division-multiplex A/N signals transmitted by a plurality of different terminals, different A/N resources need to be allocated to the respective terminals. In LTE and LTE-Advanced, control information of each terminal is always mapped to different PDCCH resources in PDCCH (physical downlink control channel) which is a downlink control channel. Focusing attention on this, LTE and LTE-Advanced prescribe that A/N resource numbers should be uniquely determined according to PDCCH resource numbers to which downlink control information is mapped. More specifically, A/N resource number nPUCCH is determined based on following equation 1 (e.g., see NPL 1).[1]nPUCCH=NPUCCH(1)+nCCE  (Equation 1)
In equation 1, A/N resource number nPUCCH is a value determined by an RB number and a spreading code number. Furthermore, NPUCCH(1) represents a constant commonly indicated to all terminals in a cell and nCCE represents a first (beginning) PDCCH resource (CCE) number among PDCCH resources to which control information of the terminals is mapped. More specifically, nCCE is given as a number (CCE index) of CCE (control channel element) defined as a resource unit of PDCCH.
The method of uniquely defining an A/N resource number using a PDCCH resource number is called an “implicit A/N resource allocation method” in the sense that it does not explicitly specify an A/N resource.”
FIGS. 1A and 1B illustrate a downlink subframe (FIG. 1A) made up of PDCCH and PDSCH (physical downlink shared channel), and uplink PUCCH resources (FIG. 1B). FIG. 1A illustrates allocation of data (PDSCH) to a plurality of terminals a to d and allocation of PDCCH including control information (e.g., allocation information) corresponding to each piece of data. On the other hand, FIG. 1B illustrates A/N resources (A/N resources in PUCCH formats 1a/1b) uniquely determined by the PDCCH resources shown in FIG. 1A. In FIG. 1B, the horizontal axis shows an RB number (UL RB index) in the uplink and the vertical axis shows a code number (PUCCH code index) in a PUCCH RB.
For example, as shown in FIG. 1B, mutually different A/N resources are associated, in a one-to-one correspondence, with respective PDCCH resources to which control information intended for terminals a to d shown in FIG. 1A is assigned. When PDSCH is allocated to the plurality of terminals a to d as shown in FIG. 1A, PDCCH resources indicating allocation information of the respective PDSCHs do not overlap with each other. That is, nCCE shown in equation 1 differs from one terminal to another. Thus, as shown in FIG. 1B, A/N resources (nPUCCH) defined in equation 1 do not overlap among the terminals either. For example, in FIG. 1B, terminal a and terminal b or terminal c and terminal d are code-division-multiplexed with the same RB respectively, and terminals a and b, and terminals c and d are frequency-division-multiplexed. Simultaneous transmission of A/N signals by different terminals is thereby made possible through code-division-multiplexing or frequency-division-multiplexing.
Aforementioned CA is a technique introduced in LTE-Advanced which enables high-speed data transmission by assigning a plurality of frequency bands (CC: component carrier) which are basic units of a communication band to one terminal. In CA, a plurality of CCs set for one terminal is made up of one PCell (primary cell) and one or a plurality of SCells (secondary cells). PCell is a CC that transmits an A/N signal, for example, when A/N signals in response to downlink data of a plurality of CCs are transmitted using only one A/N signal. Alternatively, PCell is a CC that transmits broadcast information relating to a CC that transmits A/N signals. Moreover, among a plurality of CCs set for a terminal, CCs other than PCell are SCells.
When CA is configured, since a plurality of pieces of downlink data are simultaneously transmitted, a single terminal needs to simultaneously feed back a plurality of A/N signals corresponding to a plurality of respective pieces of downlink data. To realize this feedback of A/N signals, LTE-Advanced adopts PUCCH format 1b with channel selection that can simultaneously feed back A/N signals of up to 4 bits and PUCCH format 3 that can simultaneously feed back A/N signals of 10 bits or more (e.g., see NPLs 1 and 3). When CA is configured, one of PUCCH format 1b with channel selection and PUCCH format 3 is configured in the terminal.
PUCCH format 1b with channel selection is a format that supports a method of expressing an A/N signal of a maximum of 4 bits in one symbol using a plurality of A/N resources of PUCCH formats 1a/1b and a QPSK constellation. More specifically, 4-bit A/N signals can be simultaneously transmitted using a maximum of four A/N resources and QPSK constellation (4 constellation points) of each A/N resource. For example, PUCCH format 1b with channel selection is used when two CCs are configured for a terminal.
PUCCH format 3 is a new PUCCH format whose spreading method is different from other formats. More specifically, PUCCH format 3 is a format that supports a method of increasing the number of transmittable bits per terminal by performing only spreading by an orthogonal code sequence corresponding to secondary spreading without using a ZAC sequence such as PUCCH formats 1a/1b. In the following description, a resource of one PUCCH format 3 defined by an RB and an orthogonal code sequence is called “A/N resource.”
When CA is configured, LTE-Advanced introduces ARI (ACK/NACK resource indicator) which is control information for indicating assignment of SCell to realize more flexible A/N resource allocation. The ARI is indicated by PDCCH and is information of, for example, 2 bits. FIG. 2 illustrates an example of use of ARIs when CA using three CCs (PCell, SCell1, SCell2) is configured. In FIG. 2, control information (PDCCH) relating to downlink data (PDSCH) of each CC is transmitted individually. Furthermore, ARIs are only included in control information of SCells. When downlink data is assigned to three CCs respectively, the base station specifies A/N resources of PUCCH format 3 using ARIs included in control information of two SCells out of respective pieces of control information of the three CCs.
In FIG. 2, A/N resource numbers (F3(1) to (4)) of four PUCCH format 3 A/N resources are previously indicated from a base station to a terminal through RRC control information or the like. The base station specifies, using ARIs, which A/N resource is used for the terminal among previously indicated A/N resources. However, ARIs included in control information of two different CCs (SCells) specify an identical A/N resource number. In FIG. 2, upon receiving control information corresponding to three CCs, the terminal feeds back A/N signals for downlink data (PDSCH) corresponding to the respective CCs using A/N resources of PUCCH format 3 specified by the ARIs.
Note that the terminal may not be able to detect part or whole control information transmitted from the base station. When the terminal has not been able to detect the control information of PCell, the terminal feeds back A/N signals using A/N resources of PUCCH format 3 specified by the ARI of SCell. On the other hand, when the terminal has not been able to detect control information of SCell, the terminal feeds back A/N signals using A/N resources when CA is not configured, that is, using an A/N resource (F1a/1b(1) shown in FIG. 2) of PUCCH formats 1a/1b determined by the CCE index used for transmission of control information of PCell according to equation 1.
A case has been described so far where A/N signals of a plurality of CCs are fed back in PUCCH format 3 as an example. However, even when the number of CCs configured for the terminal is 2 and PUCCH format 1b with channel selection is applied, A/N resources are selected using an ARI in the same way as shown above.
Furthermore, LTE Release 11 (Rel.11, which may also be referred to as LTE-Advanced as in the case of Rel.10) is studying to adopt ePDCCH (enhanced PDCCH) arranged in PDSCH as a new control channel different from PDCCH (e.g., see NPL 4). Unlike PDCCH transmitted by resources common to all terminals in a cell, ePDCCH is allocated to an RB indicated for each terminal FIG. 3 illustrates an example of operation of allocating downlink data (PDSCH) using PDCCH or ePDCCH. In FIG. 3, control information intended for terminals a to d is transmitted using PDCCH and control information intended for terminals e to g is transmitted using ePDCCH.